A magnetic random access memory (MRAM) wafer is generally a silicon wafer onto which is built, or disposed, a group of magnetic memory chips (also know as dies). A typical MRAM wafer might comprise thousands of magnetic memory chips. The magnetic memory chips are sectioned across the MRAM wafer in a grid pattern prior to separation from the MRAM wafer for subsequent use in electronic devices.
The silicon wafer provides a platform on which magnetic memory chips are built. Each magnetic memory chip comprises a plurality of magnetic memory arrays, and each magnetic memory array includes multiple magnetic memory cells. Magnetic memory arrays have intersecting word lines and bit lines. Magnetic memory cells, or memory cells, are formed at the intersection of word lines and bit lines. In this way, each magnetic memory array has thousands, if not millions, of memory cells.
Word lines and bit lines are routed across the memory arrays. Word lines extend along rows of memory arrays and bit lines extend along columns of memory arrays. The word lines and bit lines are referred to as conductive traces. Because the word lines and bit lines operate in combination to switch the orientation of magnetization of the selected memory cell, the word lines and bit lines are collectively referred to as write lines. To “write to the memory cell” is to switch the orientation of the magnetization of a selected memory cell. Additionally, the write lines can also be used to read the logic value stored in the memory cell.
The typical magnetic memory cell includes a layer of magnetic film, in which the magnetization is alterable, and a layer of magnetic film in which the magnetization is fixed, or “pinned”, in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer, or sense layer, and the magnetic film that is pinned may be referred to as a reference layer.
Each memory cell stores a bit of information as an orientation of magnetization. The orientation of the magnetization will assume one of two stable orientations that define the memory state of the memory cell. These two stable orientations represent logic values of “1” and “0”.
The orientation of magnetization of the selected memory cell can be changed, or switched, by the application of an external magnetic field. Generally, the external magnetic field can be created by supplying an electric current to the word line and the bit line associated with the selected memory cell. In this way, electric currents in the word and bit lines create magnetic fields that can switch the orientation of magnetization (and thus the logic value) of the selected memory cell. Since no electric power is needed to maintain the memory state of the device, the memory cells are known as non-volatile memory cells.
Preferably, only the selected magnetic memory cell is subjected to both the word line and bit line write fields. Memory cells coupled only to the particular word line preferably receive only the word line write field. Likewise, memory cells coupled only to the bit line preferably receive only the bit line write field.
The magnitudes of the word line and bit line write fields are usually selected to be low enough so that the chosen memory cell switches logic state only when subjected to both fields, and other memory cells, that are subjected to just a single write field, do not switch. The undesirable switching of a magnetic memory cell that receives only one write field is commonly referred to as “half select” switching.
In some instances, MRAM devices may experience stray or external magnetic fields that emanate from sources other than the word line and bit lines. The magnetic fields that emanate from the word lines and the bit lines are intended to write to a particular memory cell. Vector addition of stray magnetic fields to word and bit line fields can alter the switching condition of bits within the array, either preventing selected bits from switching or causing unselected bits to switch. In some instances, stray magnetic fields can have a magnitude sufficient to switch the logic state of a memory cell, even in the absence of word or bit line magnetic fields.